1. Field of the Invention
The invention relates to a method of reutilizing high-boiling compounds within an integrated chlorosilane plant for the production of semiconductor silicon comprising trichlorosilane production, Si deposition and the production of finely divided silica.
2. Description of the Related Art
In an integrated plant for producing semiconductor silicon, trichlorosilane as crude silane is produced either from metallurgical silicon and HCl or from metallurgical silicon and SiCl4/H2 in a fluidized-bed reactor. The crude silane is subsequently purified by means of distillation/purification to give trichlorosilane. Polycrystalline silicon is deposited from the purified trichlorosilane, which results in formation of, inter alia, SiCl4. The utilization of the SiCl4 (e.g. hydrogenation to form trichlorosilane or combustion to produce finely divided silica or silicic esters) is known from the prior art. The deposition of polycrystalline silicon from a mixture of chlorosilanes, in particular trichlorosilane, and hydrogen, forms not only SiCl4 but also a fraction comprising high-boiling chlorosilanes (Sirtl, J. ELECTROCHEM. SOC. 121 (1974) 919; Sirtl, Z. ANORG. ALLG CHEMIE 332 (1964) 113; Göppinger DE3024319). The term “high-boiling chlorosilanes” here refers to compounds which consist of silicon, chlorine, and also possibly hydrogen, oxygen and carbon, and have a boiling point higher than that of tetrachlorosilane (57° C./at 1013 hPa). These compounds are preferably disilanes HnCl6-nSi2 (n=0-4) and higher oligo(chloro)silanes which preferably have from 2 to 4 Si atoms and also disiloxanes HnCl6-nSi2O (n=0-4) and higher siloxanes which preferably have from 2 to 4 Si atoms, including the cyclic oligosiloxanes and their methyl derivatives. This fraction will hereinafter also be referred to as high boiler fraction.
Thus, the offgas from the Si deposition reactors comprises dichlorosilane, trichlorosilane and silicon tetrachloride together with amounts (0.001-2% by weight) which vary as a function of the deposition conditions of high-boiling chlorosilanes. In a typical composition, these high-boiling chlorosilanes comprise, for example, 50% by weight of Si2Cl6, >35% by weight of Si2HCl5, 10% by weight of Si2H2Cl4 (2 isomers), 5% by weight of Si2H3Cl3 (2 isomers) and <1% by weight of higher silanes (for example Si3Cl8).
The offgases from the deposition of polycrystalline silicon can be recycled virtually in their entirety for the production of polycrystalline silicon. The offgases are condensed and subsequently distilled. Here, the low boiler fraction, which comprises silanes having a boiling point of <40° C. under atmospheric conditions (1013 hPa) (e.g. monochlorosilane, dichlorosilane and trichlorosilane), is separated by distillation from the higher-boiling chlorosilanes and reused for the deposition of Si. Silicon tetrachloride is partly separated by distillation from the remaining mixture of silicon tetrachloride and the high-boiling chlorosilanes and is, for example, converted by hydrogenation into trichlorosilane which is in turn used in the deposition of Si. The mixture of silicon tetrachloride and the now somewhat concentrated high-boiling chlorosilanes which remains as a residue can be processed further by various methods. This high boiler fraction consists virtually entirely of disilanes (hexachlorodisilane, pentachlorodisilane, tetrachlorodisilane and trichlorodisilane) and oligosilanes (trisilanes and tetrasilanes). Due to the high purity of this fraction, it is possible to pass it to an economically beneficial use.
Thus, U.S. Pat. No. 6,846,473 describes a process for the chlorination of this high-boiling fraction with subsequent isolation of Si2Cl6 for use for semiconductor purposes.
It is also known that this high boiler fraction can be subjected to the further processing steps mentioned below to give a recyclable trichlorosilane and silicon tetrachloride. Thus, JP1-188414 (Yoshitomi, Oomure Osaka Titanium Co., Ltd. 1988) describes the recirculation of this fraction to the fluidized-bed reactor for preparing trichlorosilane.
Wakamatsu JP09-263405 Tokuyama 1996 describes the cleavage of the disilanes which have been obtained from the deposition of Si by means of HCl over activated carbon at elevated temperature.
The reaction of this high boiler fraction with silicon tetrachloride and hydrogen in a high-temperature reactor is described in US2002/0187096 (Kendig, Landis, McQuiston Dow Corning 2001).
A further possibility of cleaving these high boilers over N or P bases is described in DE3503262.
In all these processes, the high boilers are cleaved by means of hydrogen and/or HCl in order to obtain trichlorosilane and silicon tetrachloride from them. These processes in each case involve a large engineering outlay for the additional process steps.
The preparation of SiO2 powders by flame pyrolysis is known, for example, from DE2620737 and EP790213. The starting material for the silicon dioxide powder is generally silicon tetrachloride. Apart from silicon tetrachloride, methyltrichlorosilane, trichlorosilane or mixtures of these with silicon tetrachloride are also employed. Chlorine-free silanes or siloxanes can also be used. According to EP790213, the use of dimeric chlorosilanes and siloxanes is also possible.